How to Calculate Hess's Law
What is Hess's Law?
Hess's Law states that the total enthalpy change of a reaction is independent of the pathway taken — it depends only on the initial and final states. This allows calculation of ΔH for reactions that are difficult to measure directly.
Step-by-Step Guide
- 1Write the target reaction
- 2Identify a series of known reactions that sum to the target
- 3Reverse any reaction that needs to be reversed (change sign of ΔH)
- 4Multiply reactions by appropriate stoichiometric factors
- 5Sum all ΔH values
Worked Examples
Input
C + O₂ → CO₂ (ΔH=−393kJ) and 2CO + O₂ → 2CO₂ (ΔH=−566kJ)
Result
C + ½O₂ → CO: ΔH = −110kJ
Subtract half of second equation from first
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